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Forty-five gas samples have been collected from natural gas manifestations at the island of Kos—the majority of which are found underwater along the southern coast of the island. On land, two anomalous degassing areas have been recognized. These areas are mainly characterized by the lack of vegetation and after long dry periods by the presence of sulfate salt efflorescence. Carbon dioxide is the prevailing gas species (ranging from 88 to 99%), while minor amounts of N2 (up to 7.5%) and CH4 (up to 2.1%) are also present. Significant contents of H2 (up to 0.2%) and H2S (up to 0.3%) are found in the on-land manifestations. Only one of the underwater manifestations is generally rich in N2 (up to 98.9%) with CH4 concentrations of up to 11.7% and occasionally extremely low CO2 amounts (down to 0.09%). Isotope composition of He ranges from 0.85 to 6.71 R/RA, indicating a sometimes-strong mantle contribution; the highest values measured are found in the two highly degassing areas of Paradise beach and Volcania. C-isotope composition of CO2 ranges from -20.1 to 0.64‰ vs. V-PDB, with the majority of the values being concentrated at around -1‰ and therefore proposing a mixed mantle—limestone origin. Isotope composition of CH4 ranges from -21.5 to +2.8‰ vs. V-PDB for C and from -143 to +36‰ vs. V-SMOW for H, pointing to a geothermal origin with sometimes-evident secondary oxidation processes. The dataset presented in this work consists of sites that were repeatedly sampled in the last few years, with some of which being also sampled just before and immediately after the magnitude 6.6 earthquake that occurred on the 20th of July 2017 about 15 km ENE of the island of Kos. Changes in the degassing areas along with significant variations in the geochemical parameters of the released gases were observed both before and after the seismic event; however, no coherent model explaining those changes was obtained. CO2 flux measurements showed values of up to about 104 g×m−2×d−1 in both the areas of Volcania and Kokkino Nero, 5×104 g×m−2×d−1 at Paradise beach, and 8×105 g×m−2×d−1 at Therma spring. CO2 output estimations gave values of 24.6, 16.8, 12.7, and 20.6 t×d−1, respectively, for the above four areas. The total output of the island is 74.7 t×d−1 and is comparable to those of the other active volcanic/geothermal systems of Greece (Nisyros, Nea Kameni, Milos, Methana, and Sousaki). [ABSTRACT FROM AUTHOR]

Abstract Twenty gas samples have been collected from the natural gas manifestations of Milos Island, the majority of which is found underwater along its coast. Furthermore, three anomalous degassing fumarolic areas (Kalamos, Paleochori and Adamas) have been recognized on-land. Almost all the gases are CO 2 -dominated with CO 2 ranging from 88 to 99% vol for the samples taken underwater, while the on-land manifestations show a wider range (15–98%) due to air contamination. Methane reaches up to 1.0% vol, H 2 up to 3.2% vol and H 2 S up to 3.5% vol indicating a hydrothermal origin of the gases. The isotope composition of He points out to mantle contributions up to 45%, while the C-isotope composition of CO 2 (from −1.9 to +1.3‰ vs. V-PDB with most of the values around −0.5‰) suggests a prevailing limestone origin. Isotope composition of CH 4 , ranging from −18.4 to −5.0‰ vs. V-PDB for C and from −295 to +7‰ vs. V-SMOW for H, points to a geothermal origin with sometimes evident secondary oxidation processes. Additionally, CO 2 -flux measurements showed high values in the three fumarolic areas (up to 1100, 1500 and 8000 g/m2/d at Kalamos, Paleochori and Adamas respectively) with the highest CO 2 -flux values (up to about 23,000 g/m2/d) being measured in the sea at Kanavas with a floating chamber. The south-western part of the island was covered with a lower density prospection revealing only few anomalous CO 2 flux values (up to 650 g/m2/d). The total output of the island (30.5 t/d) is typical of quiescent closed-conduit volcanoes and comparable to the other volcanic/geothermal systems of the south Aegean active volcanic arc (Nisyros, Kos, Nea Kameni, Methana and Sousaki). [ABSTRACT FROM AUTHOR]